1. Field of the Invention
The present invention relates to a marine vessel which includes a steering mechanism for turning a hull thereof, and a marine vessel running controlling apparatus for such a marine vessel.
2. Description of the Related Art
An exemplary propulsion system provided in a marine vessel such as a cruiser or a boat for a leisure purpose is an outboard motor attached to a stern (transom) of the marine vessel. The outboard motor includes a propulsion unit provided outboard of the vessel. A steering mechanism is attached to the propulsion unit. The propulsion unit includes an engine as a drive source and a propeller as a propulsive force generating member. The steering mechanism horizontally turns the entire propulsion unit with respect to a hull of the marine vessel. When the steering mechanism is driven to turn the propulsion unit, the steering angle of the steering mechanism (a direction in which the propulsion unit generates a propulsive force) is changed, whereby the hull is turned.
A control console for controlling the marine vessel is provided on the hull. The control console includes, for example, a steering operational section for performing a steering operation, and a throttle operational section for controlling the propulsive force generated by the propulsion unit. The steering operational section includes, for example, a steering wheel as an operational member to be operated by an operator of the marine vessel. The steering wheel is mechanically connected to the steering mechanism via a wire or a hydraulic mechanism. Therefore, the steering mechanism is driven by operating the steering wheel to change the steering angle. Since the steering wheel and the steering mechanism are mechanically connected to each other, a relationship between the operation amount of the steering wheel and the steering angle is constant irrespective of the traveling speed of the hull.
An exemplary relationship between a stepwise change in the steering angle and the turning speed (yaw rate) of the marine vessel at a given traveling speed is shown in FIG. 1. Where the stepwise change in the steering angle is defined as an input signal and the yaw rate is defined as an output signal, a transfer function G(s) defining a relationship between the input signal and the output signal is approximately given by the following expression (1). This transfer function G(s) is a primary delay model called “Nomoto Model”.G(s)=K/(T·s+1)  (1)
wherein s is a Laplacian, T is a time constant, and K is a gain.
If the traveling speed (or an engine speed N (rpm) as an alternative index) varies as shown in FIG. 2, the response of the yaw rate to the stepwise change in the steering angle also varies. That is, the time constant T and the gain K vary depending on the traveling speed. Here, the time constant T has a smaller variability than the gain K with respect to the traveling speed. Therefore, only the gain K will herein be discussed.
Although depending on the shape of the hull, the gain K increases with an increase in the traveling speed (with an increase in the engine speed N). Therefore, a higher yaw rate is provided in response to a change in the steering angle when the marine vessel is in a higher speed traveling state (e.g., when the marine vessel travels in the ocean) than when the marine vessel is in a lower speed traveling state (e.g., when the marine vessel travels at a lower speed in the vicinity of a docking site).
When the steering angle is changed by a certain degree, a lower yaw rate is provided to turn the hull more slowly in the lower speed traveling state than in the higher speed traveling state. If the operator desires to sharply turn the hull in the lower speed traveling state, the operator has to operate the steering wheel intentionally by a greater operation amount to increase the steering angle. In the higher speed traveling state, on the other hand, a higher yaw rate is provided to turn the hull more sharply than in the lower speed traveling state. Therefore, if the steering wheel is operated in the higher speed traveling state by the same operation amount as in the lower speed traveling state, there is a possibility that the hull is turned more quickly than intended by the operator. If the operator desires to slightly turn the hull in the higher speed traveling state, the operator has to operate the steering wheel intentionally by a smaller operation amount to reduce the steering angle.
Since the relationship between the steering angle and the yaw rate varies depending on the traveling speed, a relationship between the operation amount and the yaw rate also varies depending on the traveling speed. Therefore, a higher level of marine vessel maneuvering skill is required for the operator to perform a steering operation intentionally in different ways depending on the traveling speed. Hence, an unskilled operator of a leisure boat or the like cannot easily perform the steering operation.
Therefore, if it is possible to change the steering angle by a relatively great degree to increase the yaw rate in the lower speed traveling state and to change the steering angle by a relatively small degree to reduce the yaw rate in the higher speed traveling state, the operator can perform the steering operation without consideration of the traveling speed. Thus, even the unskilled operator can easily and properly perform the steering operation. With the steering wheel mechanically connected to the steering mechanism as described above, however, the relationship between the operation amount of the steering wheel and the steering angle cannot be changed according to the traveling speed of the hull.
Electric steering apparatuses for marine vessels are proposed in US 2005/0282447A1, US 2007/0066156A1 and US 2007/0066154A1. In these electric steering apparatuses, the operation amount of the steering wheel is detected by a potentiometer or the like, and the steering mechanism is driven according to a target steering angle calculated based on the detected operation amount. With this arrangement, the relationship between the operation amount of the steering wheel and the steering angle can be changed according to the traveling speed. Therefore, the relationship between the operation amount and the yaw rate (marine vessel maneuvering characteristic) with respect to the traveling speed is supposedly improved by properly setting a relationship between the operation amount and the target steering angle according to the traveling speed. Further, US 2007/0066154A1 proposes that a characteristic defining the relationship between the operation amount and the target steering angle is preliminarily provided and the target steering angle is calculated based on the characteristic in consideration of a marine vessel traveling state.
Boat builders and users are permitted to select different types of propulsion units, steering mechanisms and hulls in any desired combinations. Therefore, a relationship between the steering angle and a marine vessel turning behavior varies among different marine vessels. For implementation of the invention disclosed in US 2007/0066154A1, it is necessary to preliminarily define a characteristic suitable for each of the marine vessels.
However, there are an infinite number of combinations of the propulsion units, the steering mechanisms and the hulls, making it impractical to preliminarily provide optimum characteristics for the respective combinations.
In addition, the relationship between the steering angle and the turning behavior varies depending on the number of crew members and/or passengers and the amount (weight) of cargo. It is practically impossible to preliminarily provide the optimum characteristics in consideration of such conditions.